In the roster of obscure yet highly successful ecological niches, there is one that knocks my socks off. There are, apparently, flies that specialize on feeding on the bodily secretions of dead or dying honeybees. As in: spider catches bee, fly smells bee struggling to escape spider, and fly swoops in to lap up delicious juices of a bee engaged in mortal combat, stealing from the spider’s meal.

I love how one of those cheeky flies lands right on the spider.

But this is not the niche that floored me. Or at least, not the only one.

There is a plant that specializes in duping these flies by emitting the scent of fake dying bee from its flowers. They trap them inside for a good 24 hours with nothing to eat -- not even pollen or nectar. They release them only after the flies have had ample time to crawl all over the flower’s naughty bits looking for food or a way out and acquire a sticky packet of plant sperm (which, in case you didn’t know, is what’s inside pollen.)

When they emerge, weakened and hungry, they may fly like idiots right into the next deceptive flower, only to be trapped all over again. In the process, they may leave that sticky packet behind, doing the flowers’ evil fraudulent sexual bidding and unwittingly making more fake dying bee flowers.

Ain’t nature grand?

The botanical star of this story is the plant Ceropegia sandersonii – the parachute flower. Want to see how it got its name?

Physical mimicry is only one way flowers sucker insects. They can also use chemicals to trick pollinators into working for free. The indescribably cute western fairy slipper orchid that lives in the Rocky Mountains above Denver and Boulder uses such a feint, luring bumblebees with an enticing smell and fake anthers and nectaries, but offering no actual nectar or pollen as a reward for a visit. However, the scent varies greatly from flower to flower. As a result, the pollinator can’t learn to associate one particular scent with the deception, and the bumblebees fall for it again and again.

Scientists knew that the genus Ceropegia employed deception to get flies into flowers, but they didn’t know exactly how. They did know that the flies that most frequently pollinated Ceropegia flowers were kleptoparasitic, that is, that stole their food from other animals by feeding on the body fluids – usually leaked from mandible or sting glands when a bee tries to bite or sting its attacker -- that seeped from spiders’ or other insect predators' struggling or recently expired prey.

Only females of these flies -- often in the genus Desmometopa -- are kleptoparasitic (just as only female mosquitoes suck blood) because they need the protein in these secretions to make their eggs. It's as if there were mosquitoes that specialized in swarming to and lapping up the blood that seeped from a deer killed by a mountain lion. Scientists had assumed that these kleptoparasitic flies used the volatile odors (also called alarm pheromones) emitted from ensnared bees to find them, which suggested that the parachute flower could be imitating those smells.

The scientists hypothesized that if Ceropegia flowers were mimicking Desmometopa bee food, they would attract such flies, the flowers would emit unusual compounds or blends of compounds not normally found in flowers, and the compounds would overlap with those emitted by struggling honeybees.

Indeed, the most common visitors to parachute flowers were kleptoparasitic Desmometopa. Inside experimental flowers, mostly female Desmometopa appeared, consistent with the idea the flowers are mimicking a food source only the ladies find irresistible. And 60% of the chemical compounds emitted by parachute flowers overlapped with the volatiles released by European and South African honeybee subspecies under “simulated attack”. Moreover, the scents emitted by the bees were highly variable, and the flowers mimicked this too by greatly varying their scent.

One of the chemicals emitted by honeybees -- an anesthetic called 2-heptanone -- targets arthropod attackers too small to be stung. This stunned me because I'd never heard anyone say this before: honeybees are effectively releasing knockout gas. And so did the parachute flower.

Electrophysiological measurements of Desmometopa flies also found that almost half of the compounds found both in parachute flowers and struggling bees were picked up by the flies’ antennae. Four of the compounds that registered on Desmometopa radar also attracted them in their pure chemical forms, away from flower or bee, and they did so within seconds, consistent with the idea these flies are responding to an ephemeral food source. The combination of these four compounds is not known to be emitted by any other flower but the parachute flower, or any other insect but the honeybee in extremis.

ABOUT THE AUTHOR(S)

Jennifer Frazer

Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.

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